Temperature and high-pressure dependent X-ray absorption of SmNiO3 at N-Ki and Sm-L3 edges

NESTOR E. MASSA; ALINE Y. RAMOS; HELIO C. N. TOLENTINO; NARCIZO M. SOUZA-NETO; JAIRO FONSECA, JR.; JOSE A. ALONSO

Cmapinas

Congreso; LNLS 25th Annual Users? Meeting (RAU); 2015

Laboratorio Nacional de Luz Sincrotron

The primary consequence of applying quasihydrostatic pressure P to a solid is the reduction of interatomic distances, the progressive bond shortening, and the modification of superexchange M-O-M angles. Pressure change as thermodynamic external dynamical variable is a unique tool for probing the relationship between structural sublattice distortions and electronic properties by triggering increments in band hybridization and the eventual emerging into a metallic state by valence and conduction band superposition. SmNiO3 is a member of the family of undoped compounds RNiO3 (R=Rare Earth) in which a sharp temperature driven insulator to metal phase transition takes place. The transition from high temperature orthorhombic metallic to an insulating charge disproportionate (2Ni3+ → Ni3+δ+Ni3−δ) monoclinic distorted phase is consequence of the Ni-O bond length increase and a simultaneous decrease of the Ni-O-Ni angle. SmNiO3, TIM = 400 K, also has a paramagnetic insulating phase, and orders magnetically below TN~205 K. A negative slope, -TMI/dP, is shared by all RNiO3 (R=Rare Earth≠ La.). We report XANES and EXAFS measurements of SmNiO3 from 20 K to 600 K and up to 38 GPa at the Ni K- and Sm L3- edges. They were performed at the LNLS in the D06A-DXAS energy dispersive beamline. Increasing pressure induces Ni-O-Ni angle increments toward more symmetric Ni3+ octahedra of the rhombohedral R¯3c space group (metallic LaNiO3). A multiple component pre-edge tail accounts for 1s transitions to 3d-4p states while a post-edge shoulder raises distinctively smoother due to the reduction of electron-phonon interactions as Ni 3d and O 2p orbitals overlap. Pressure dependent room temperature Ni white line peak energies show an abrupt 3.1 GPa valence discontinuity due to non-equivalent Ni sites with Ni3+δ + Ni3-δ charge disproportionate in the monoclinic distortion turning at TMI into Ni3+ of the perovskite distorted orthorhombic Pbnm metal oxide phase. At 20 K, still distinctive, this turns smoother due to localization. Octahedral bond contraction at 300 K and 20 K show breaks in its monotonic increase at the different structural changes up to 38 GPa. Sm L3-edge does not show distinctive behaviors either at 300 K or 20 K up about 35 GPa but bonds contraction undergoes a critical distortion at intermediate pressures. Our findings suggest that the white line pressure dependent anomaly may be used as a reliable alternative for delineating pressure phase diagrams in RNiO3 (R=Rare Earth).